Voice format: Telnyx.Ultra.<voice>
Sub-100ms latency. 36 languages.
REST only — Ultra is not available over public WebSocket.
Voice Samples
Expressive Mode for AI Assistants
Enable Expressive Mode when using an Ultra voice with AI Assistants. The assistant can select emotion tags and nonverbal cues from the conversation context without hard-coding each expression in the system prompt.
For example, an assistant can use an excited tone for good news, respond empathetically to a problem, insert [laughter] in a friendly exchange, and keep routine information neutral.
Enable in the portal
- Open the assistant in the Telnyx Portal.
- Under Voice Settings, select a
Telnyx.Ultra.* voice.
- Enable Expressive Mode.
- Save the assistant.
Enable via API
Set expressive_mode: true in the assistant’s voice_settings:
expressive_mode requires a supported expressive voice. Use a Telnyx.Ultra.* voice for Ultra Expressive Mode.
SSML Emotions
Ultra supports inline SSML emotion tags. Place the tag before the text:
Primary emotions: angry, excited, content, sad, scared.
Additional: happy, enthusiastic, curious, calm, grateful, affectionate, sarcastic, surprised, confident, hesitant, apologetic, determined, frustrated, disappointed.
Omitting the tag = neutral delivery. Use sparingly — Ultra interprets emotional subtext from the text itself.
Pauses and Pacing
Insert <break time="Xs" /> inline to add a pause. Use short pauses for pacing and emphasis; longer pauses can make a caller think the call dropped.
Repeat an ID slowly by splitting it into groups:
Pause while looking up information:
Emphasize important information:
Combine a pause with an emotion:
As a starting point, use 0.2–0.4 seconds for slight emphasis or dictation, 0.5–0.8 seconds for topic transitions, and 1–1.5 seconds for a lookup or significant shift. Avoid pauses longer than 2 seconds in a conversation.
Nonverbal Cues
Insert [laughter] inline for natural laughing:
Language Support
Set language_boost to improve pronunciation for the target language:
Arabic, Bengali, Bulgarian, Chinese, Czech, Danish, Dutch, English, Finnish, French, German, Gujarati, Hebrew, Hindi, Indonesian, Italian, Japanese, Korean, Malay, Marathi, Māori, Norwegian, Polish, Portuguese, Punjabi, Romanian, Russian, Slovak, Spanish, Swedish, Tamil, Telugu, Thai, Turkish, Ukrainian, Vietnamese.
REST API
Fields
Response
Default (binary_output): chunked audio bytes with Content-Type: audio/mpeg.
With output_type: "base64_output": JSON with base64-encoded audio.
With output_type: "audio_id": JSON with an audio_url for deferred retrieval.
See also
xAI Grok is the second TTS provider supporting Expressive Mode. For Grok voice options, see Grok Voices. Note: Grok voices have higher latency than Ultra. 
